71 valiant engine build, which carb to use

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Penstarpurist

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So a few months ago we had to pull the 318 engine out of my son's 71 valiant 2dr. The thrust bearing had failed, wiped out the crankshaft. Block was magnafluxed and deemed worthy for a rebuild. Original block to the car, we had a crankshaft out of a 68 318 turned, the block had to be punched .40 over, he bought the forged scat rods, kieth black hyperutetic pistons, mahl bearings throughout, and of course the edelbrock aluminum head, air gap intake, cam, lifters and timing set kit. The machine shop did a full assembly, file fit everything. Line bored, balanced, etc. The question is what is the best size carburetor to use. He has fairly new 600cfm edelbrock avs, the edelbrock kit came with a free 800cfm Avs carb. I'm thinking the 600 is too small and the 800 probably too big. The kit came with choice of a 600 or 800, since he had a new 600 he opted for the 800. Cam size is @.50 234int/244exh advertised is 308int/318exh 2.02 intake valves 1.6 exhaust valves the machine shop is estimating a 10.1 compression ratio. Any thoughts on proper carb size? I ran the standard formula for it and it lands near 600 cfm with that, but not knowing exact Volumetric Efficiency, rpm max, I could only put in assumed numbers. I would think it would be better suited with at least a 650 cfm. Lol, I told him efi would be the best option, but he likes the old school carburetor.
 
put the 800 on and don't look back...
I do like the looks of those demon carbs, I know they perform well. I really like the idea of dropping the 800 cfm edelbrock on there and not looking back, especially when it is a new in the box and on the shelf part. Just wanted to be sure it's not going to overfuel it or starve it with the 600 cfm.
 
I do like the looks of those demon carbs, I know they perform well. I really like the idea of dropping the 800 cfm edelbrock on there and not looking back, especially when it is a new in the box and on the shelf part. Just wanted to be sure it's not going to overfuel it or starve it with the 600 cfm.
I have 1200 cfm's on my 318.... (a pair of 600's), and it's nowhere near the motor you are working on....
 
I have 1200 cfm's on my 318.... (a pair of 600's), and it's nowhere near the motor you are working on....
Dang, I can't argue with that. If you're running dual quad with 1200cfm on a mild 318 with no Ill effects then definitely the 800 cfm would be good to go on ours. Thanks for the info.
 
I have used two of the Holley made Street Demon carbs, which are a 'loose' copy of the TQ. Demon was probably a good name for them. The quality is quite poor & from what I remember met rod changes were fiddly. One carb performed quite well, the other had a bog when the secondaries opened that no amount of tuning or jet change could fix. No bog with the original QJ carb. If these carbs had the air valve dashpot like the original the TQs had, it would have given more tuning choices. Maybe omitted for cost?

If the engine is to be street driven, the cam is a poor choice for a 318. The duration numbers at 050 are pretty big, but the huge advertised duration numbers will guarantee a pretty rough idle.

Put an 800 TQ on it.
 
Who manufactured that cam and what's the lift and LSA? Those advertised duration numbers look like they're measured at .002. In the modern fast rate cams, those numbers would equate to advertised numbers somewhere in the 276-292 degrees of duration zone. I hope he's got some fairly deep gears, the compression is a little lower than I'd like to see with those kinds of cam numbers.
 
Who manufactured that cam and what's the lift and LSA? Those advertised duration numbers look like they're measured at .002. In the modern fast rate cams, those numbers would equate to advertised numbers somewhere in the 276-292 degrees of duration zone. I hope he's got some fairly deep gears, the compression is a little lower than I'd like to see with those kinds of cam numbers.
The entire top end kit is all edelbrock, it's their top end kit, so everything is a matched to play well together kit, hopefully. The car is being built for a hot street/strip set up. As for gears, currently its a 3.55 in an 8 3/4 rearend. The 904 transmission is getting swapped out for a 727 with a shift kit and 1800-2000 stall convertor.
 
Since you have both I would run the 625 for break in (especially if it s a know good carb) and street driving until you can get to the track and try both to see what the car like better. Install a wide band to help tune either.
 
You already own the best carb for that build. 650 AVS2.
We dont yet have a 650 avs2, we have a 600cfm and an 800cfm, we would have to buy the 650cfm. Which is not a problem, at this point there's so much money into it that buying a new carb that will work right for the build is just a drop in the bucket. That said, if one of the carbs we have would be a good choice that would be great.
 
We dont yet have a 650 avs2, we have a 600cfm and an 800cfm, we would have to buy the 650cfm. Which is not a problem, at this point there's so much money into it that buying a new carb that will work right for the build is just a drop in the bucket. That said, if one of the carbs we have would be a good choice that would be great.
Ok, I have a 69 360 Dart that I've had a Holley 650 DP, a 1406 Edelbrock on. But lately I just put a 1906 650 AVS2 on. What a difference, Throttle response is unreal.
 
We dont yet have a 650 avs2, we have a 600cfm and an 800cfm, we would have to buy the 650cfm. Which is not a problem, at this point there's so much money into it that buying a new carb that will work right for the build is just a drop in the bucket. That said, if one of the carbs we have would be a good choice that would be great.
the factory put 950 cfms on a 340 on the show room, not even headers. Factory also put 800 cfm Thermoquads on stock 318's with single exhaust. I've done carb swaps at the track for testing. Click the link right below in my post and watch my very mild, low compression 318 with 318 heads run with 1200 cfms of carb on top, no headers, 2.76 gears and stock converter. My opinion again, 800 and don't look back.
 
the factory put 950 cfms on a 340 on the show room, not even headers. Factory also put 800 cfm Thermoquads on stock 318's with single exhaust. I've done carb swaps at the track for testing. Click the link right below in my post and watch my very mild, low compression 318 with 318 heads run with 1200 cfms of carb on top, no headers, 2.76 gears and stock converter. My opinion again, 800 and don't look back.
You can put a 1350 Dominator on a 318, doesn't mean it's using 1350 CFM or that it is efficient.
 
As said before, it's only going to flow as much air as what the engine will pull. The annular boosters in the AVS2 give it excellent tunability and street manners, though.
 
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the factory put 950 cfms on a 340 on the show room, not even headers. Factory also put 800 cfm Thermoquads on stock 318's with single exhaust. I've done carb swaps at the track for testing. Click the link right below in my post and watch my very mild, low compression 318 with 318 heads run with 1200 cfms of carb on top, no headers, 2.76 gears and stock converter. My opinion again, 800 and don't look back.
Don't you understand what carburetors are rated at and the signal given by the engine you're not flowing 1200 CFM.
You edited your post and i guess i wasn't fast enough to login and quote you, you said 6 pack at 1st not 950cfm, which is more like 955cfm
Just imagine if 1200 cfm were being digested ...you're only running 13's ??
The 318 thermo... well they made many cfm rating for those and the primary is so tiny 'stronger signal=more mixing of fuel and air... that it can run that big carb, BUT it is not even opening the secondary all the way... that air door is for a reason.
You're not an idiot, but you have a sort of confident incompetence about you. You would run faster with one 4 brl than the intake and dos 600's , even a couple 390's.
Always amazed how when something runs good enough for somebody...they think it's perfect. They could be horsepower ahead and better milage.
 
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The entire top end kit is all edelbrock, it's their top end kit, so everything is a matched to play well together kit, hopefully. The car is being built for a hot street/strip set up. As for gears, currently its a 3.55 in an 8 3/4 rearend. The 904 transmission is getting swapped out for a 727 with a shift kit and 1800-2000 stall convertor.
You should rethink your converter choice. It will be an absolute dog with that cam. You really need at minimum , 3500 stall. 4000 plus would be better.
 
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